A hydraulic circuit for a construction machine in the related art, as illustrated in FIG. 1, includes a variable displacement hydraulic pump (hereinafter referred to as a “hydraulic pump”) 1 connected to an engine (not illustrated) or the like; at least two hydraulic actuators 2, 3, and 4 driven by hydraulic fluid that is supplied from the hydraulic pump 1; control valves 6, 7, and 8 installed in a center bypass path 5 of the hydraulic pump 1 and shifted to control a start, stop, and direction change of the hydraulic actuators 2, 3, and 4; a parallel flow path 9 having inlets branched and connected to predetermined positions on an uppermost stream side of the center bypass path 5 and outlets connected to inlet ports of the control valves 6, 7, and 8; a first orifice 11 installed in a predetermined position of a first path 10 having an inlet branched and connected to a predetermined position of the parallel flow path 9 and an outlet connected to an inlet port of the control valve 7; and a second orifice 13 installed in a predetermined position of a second path 12 having an inlet branched and connected to the predetermined position of the parallel flow path 9 and an outlet connected to an inlet port of the lowermost downstream side control valve 8.
If an operation lever (RCV) (not illustrated) is operated to operate the hydraulic actuators 2, 3, and 4 for a combined work, pilot signal pressure from a pilot pump (not illustrated) is applied to the control valves 6, 7, and 8 to shift spools thereof, and thus it becomes possible to control the hydraulic fluid that is supplied from the hydraulic pump 1 to the hydraulic actuators 2, 3, and 4.
In this case, if the control valves 6 and 7, the control valves 6 and 8, or the control valves 7 and 8 are shifted by the applied pilot signal pressure, for example, if the control valves 6 and 7 are shifted, the hydraulic fluid of the hydraulic pump 1 is supplied to the hydraulic actuator 2 via the upstream side control valve 6 of which the spool is shifted, and the hydraulic fluid of the hydraulic pump 1 is supplied to the hydraulic actuator 3 via the parallel flow path 9, the first path 10, and the downstream side control valve 7 of which the spool is shifted.
In this case, the center bypass path between the upstream side control valve 6 and the downstream side control valve 7 is closed by the shifting of the upstream side control valve 6, and thus the hydraulic fluid of the hydraulic pump 1 is supplied to the inlet port of the downstream side control valve 7 only through the parallel flow path 9. Further, since the hydraulic fluid of the hydraulic pump 1 is supplied to the inlet port of the downstream side control valve 7 via the first orifice 11 that is installed on the first path 10, an excessive pressure loss occurs during the combined work, and thus energy efficiency is decreased.